The invention relates to an X-ray examination apparatus, including a high voltage generator for an X-ray tube and a number of additional power supply units for further components of the X-ray examination apparatus, there also being provided a power factor correction circuit.
The use of a power factor correction circuit in an X-ray examination apparatus is known from international patent application WO-A 96/17260. Therein, power factor correction is applied so as to correct the phase difference between the voltage and the current on the input leads, which difference is caused by the input impedance of the apparatus. The known X-ray examination apparatus includes a central power supply unit wherefrom the supply voltages for the X-ray tube and the further components are derived. The power factor correction circuit forms part of said central power supply unit. However, in practice there will often be a situation where individual power supply units are provided for the further components in the X-ray examination apparatus; in that case the method of applying the power factor correction as disclosed in the cited publication cannot be used.
Nowadays it is customary to apply power factor correction to the power supply for a variety of apparatus, particularly when a non-sinusoidal, notably a peak or pulse-shaped current is derived therefrom; this correction will be referred to hereinafter as PF correction. The power factor or PF is the ratio of the actual power to the apparent power. When a phase difference exists between an alternating voltage and an alternating current or a peak or pulse-shaped current, the PF may deviate significantly from the value xe2x80x9c1xe2x80x9d and may even become negative, depending on the phase difference or the pattern of the current variation. In the case of a sinusoidal voltage and current variation, the power factor is represented by the cosine of the phase difference between the two of them. In the case of a peak or pulse-shaped current variation, the current shape can be subdivided into a number of components of different frequency, i.e. the harmonics, which individually exhibit a phase difference with respect to the alternating voltage. The component having the main frequency delivers the actual power whereas the other components are responsible for the losses occurring. A PF correction circuit in principle matches the current variation with the voltage variation, that is to say, in such a manner that the PF assumes substantially the value xe2x80x9c1xe2x80x9d. Consequently, after PF correction a substantially higher actual power can be extracted from the mains. When such a PF correction circuit is incorporated in the high voltage generator, therefore, a higher actual power will become available to this generator. However, this does not yet offer optimum use of the available power. If PF correction were also applied to all additional power supply units, the power available to the high voltage generator could be further increased. However, it is comparatively expensive to provide all such power supply units with a PFC circuit; moreover, the overall design of the complete apparatus must be attuned to such modified power supply units.
It is an object of the invention to mitigate this problem at least significantly and to provide an X-ray examination apparatus in which power factor correction is realized for the entire system, without providing the individual additional power supply units with a power factor correction circuit.
To this end, the X-ray examination apparatus of the kind set forth according to the invention is characterized in that there is provided an input circuit which is to be connected to the mains and includes a current measuring circuit, the high voltage generator and the additional power supply units being connected to said input circuit, the high voltage generator being provided with a rectifier unit whereto there is connected the power factor correction circuit which is controlled via the input circuit in such a manner that the variation of the current drawn via the rectifier unit, together with that drawn by the additional power supply units collectively, is mainly sinusoidal and in phase with the main voltage.